Upgrading Lignite Through Drying And Pyrolysis With Microwave Irradiation

An increasing number of investigations have recently been conducted into the upgrading of lignite. A promising upgrading approach involves microwave heating, which offers unique features, such as non-contact, selective and volumetric heating. However, the upgrading mechanisms of microwave irradiation on specific physicochemical properties of lignite have been seldom investigated in the literatures. Inorganic ions were used to improve the permittivity of lignite to enhance the microwave dewatering. Microwave susceptors were employed to accelerate the heating rate of lignite to enhance the pyrolysis. Lignite was continuously upgraded in a tunnel-type microwave irradiation system to improve its slurryability.Effectes of different factors, including microwave power, diameter-to-height ratio of lignite pile, particle size and initial moisture content of lignite on the microwave dewatering process of lignite were studied. NaCl was for the first time used to improve the permittivity of lignite, thus enhancing the microwave dewatering rate. The dielectric loss factor of lignite with NaCl increased, enhancing the conversion of electromagnetic energy to thermal energy. As a result, the moisture diffusion coefficient of lignite increased. Furthermore, the dewatering rate of lignite increased as an Asymptote rule growth with the increasing NaCl content. The moisture content of lignite decreased with the addition of NaCl under a constant effective microwave power and irradiation time. The pore fractal dimension of dewatered lignite pores decreased with the addition of NaCl due to the accelerated dewatering rate. The promoting effect of Na+ on microwave dewatering was better than those of K+ and Ca2+, whilst the promoting effect of Cl-was better than that of SO42-. This phenomenon may be because Na+ and Cl- with small hydrated radii exhibit high movement speeds in electromagnetic fields.Physicochemical properties of the removed water produced from the microwave upgrading process of lignite were analysed. Results showed that total organic carbon and chemical oxygen demand increased with increased microwave time due to the decomposition of functional groups in lignite. Organic matters in removed water mainly consists of aromatic, aliphatic, and oxygen functional groups. During the microwave process, inorganic anions (SO42-, F- and Cl-) and cations (Na+, Ca2+ and K+ etc.) gradually leached out from lignite. Concentrations of the dominant ions initially increased and then decreased due to the competitive effect of leaching rate of ions and removal rate of water. The electrical conductivity initially increased and then decreased, whereas surface tension and pH monotonously decreased.To upgrade lignite by reducing moisture and volatile matter content, activated carbon and graphite were employed to accelerate heating rate via microwave irradiation. The oxygen functional groups of the upgraded lignite decreased, whereas coal rank increased. Furthermore, the atomic ratio of oxygen to carbon and the molar ratio of carbonyl to the aromatic groups of the lignite upgraded with the aid of activated carbon were both lower than those of lignite upgraded with the inclusion of graphite. On the contrary, the aromaticity and aromatic-to-aliphatic ratio of the first type of upgraded lignite were both higher than those of the second type. The hydrophilic functional groups on the surfaces of lignite upgraded with the aid of activated carbon were lower than those on the surfaces of lignite upgraded with graphite.The coke (i.e. pyrolysed coal) which is preferred as good microwave susceptor given that it is inexpensive, easily available and with high dielectric permittivity was used to accelerate the microwave pyrolysis of lignite. The physicochemical properties of the pyrolysed lignite were investigated through Fourier transform-infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, scanning electron microscopy, N2 adsorption porosimetry and contact angle analysis. Under the same microwave power and irradiation time, with the aid of coke, the temperature of lignite increased, the molar ratio of methyl to methylene group, the aromaticity and the superficial π-π* content of lignite increased, as a result, the volatile matter of lignite decreased.To meet industrial requirements for coal water slurry volume production, lignite was continuously upgraded in a tunnel-type microwave irradiation system to improve its slurryability. The total content of acidic oxygen functional groups of upgraded lignite decreased, the contact angle increased, the hydrophilicity index decreased, and the superficial chemical additive adsorption amount increased. The upgraded lignite was distributed in a wider particle size range. As a result, the coal water slurry property of upgraded lignite was improved.